Plastic containers are widely used as packaging containers in various fields, for example, in the fields of food and medical products because of their various advantageous characteristics such as ease of molding, light weight, and low cost. Such plastic containers, however, suffer from various problems. For example, plastic containers allow permeation of low molecular gases such as oxygen, carbon dioxide, and water vapor and allow adsorption of low molecular organic compounds onto their inner surfaces. Various attempts have been made to solve these problems. For example, as an attempt to reduce the gas permeability of a plastic container, a technique of stacking an ethylene-vinyl alcohol copolymer resin layer having gas barrier properties on a matrix resin layer made of a polyolefin resin is employed. As another example of such an attempt, a technique of using a resin blend of a polyolefin resin and a gas barrier resin is employed.
Ethylene-vinyl alcohol copolymers have drawbacks such as very poor gas barrier properties under high humidity conditions, although they exhibit relatively good oxygen barrier properties under dry conditions. In addition, the use of these resins can reduce the gas permeability to some extent but if not sufficient to achieve better gas barrier properties required.
In-mold labeling is well known as a technique for improving the gas barrier properties of containers. In in-mold labeling, a label with a heat seal layer is set in a mold before blow molding or injection molding. The heat seal layer of the label is fused to the surface of a molded body by the heat of a resin used for the molding of the body, and thus labeling is performed simultaneously with the molding.
Patent Literature 1 discloses an in-mold labeled container and a production method thereof. Patent Literature 1 discloses a method for performing in-mold labeling using a label including a barrier layer composed of a metal foil layer or a thermoplastic resin layer. Patent Literature 1 also discloses a polyethylene terephthalate film including deposited silicon oxide as a gas barrier layer.
However, in such a conventional in-mold label, a gas barrier layer may suffer from defects such as cracks and pinholes when exposed to physical stresses such as deformation and impact. Therefore, the gas barrier properties may be insufficient when used in practice. For example, when an in-mold labeled container is used as a food packaging material, it is exposed to physical stresses of varying magnitude in the processes of food filling, transportation, displaying, and consumption. In addition, the barrier properties of the container may be reduced due to thermal contraction by heating during in-mold labeling. There has thus been a demand for an in-mold label capable of maintaining good gas barrier properties even when exposed to physical stresses.